You can set up 'anchors' in the inspector - anchor an object to the left and to the right, to have it horizontally centered
It might be somewhat tricky depending on other layout-objects - but that's the way it can be done, the starting point
You can have the layout objects on the left side anchored to the left only, the objects on the right side anchored to the left and to the right, etc.
If you want all your windows to always be centered then an on layout enter script trigger would work for you. The script itself would set your left layout window edge to (Get (ScreenWidth) - Get(WindowWidth))/2.
So if the screen width is 1280, your window is 600 the difference is 680 and the left edge would be 340
That's what I do but I am relative novice and there may be a more elegant way to do this consistently.
You could create a reusable script that you could use when needed in your navigation scripts to do this as well.